Attention has recently been focused on fuel cells as an energy supply source which has a high power generation efficiency and theoretically yields only water as a reaction product, while being excellent in environmental friendliness. Depending on species of electrolytes employed, such fuel cells are roughly classified into low-temperature operating fuel cells such as those of alkali, solid polymer, and phosphate types, and high-temperature operating fuel cells such as those of molten carbonate and solid oxide types. Among them, polymer electrolyte fuel cells (PEFCs) using a solid polymer s their electrolyte, which can attain a high density/high output in a compact structure while being operable in a simple system, have been widely studied not only as a stationary distributed power supply but also as a power supply for vehicles and the like, and have been greatly expected to come into practical use.
One of such PEFCs is a direct alcohol fuel cell which directly uses an alcohol as its fuel, in which a direct methanol fuel cell (DMFC) using methanol as its fuel has been known in particular. When methanol and water are supplied to an anode (fuel electrode) of the DMFC, methanol is oxidized by water, so as to generate a hydrogen ion. The hydrogen ion migrates through the electrolyte to a cathode (air electrode), thereby reducing oxygen fed to the cathode. According to these redox reactions, a current flows between both electrodes.
Thus, the direct alcohol fuel cell can directly use alcohol, which is a fuel, for power generation without modifying it into hydrogen and the like, and thus has a simple structure without necessitating a separate device for fuel modification. Therefore, the direct alcohol fuel cell can be made smaller and lighter very easily, and can favorably be used for a portable power supply and the like.
Each of the anode and cathode of the PEFC is constructed, for example, by two layers, i.e., a catalyst layer to become a reaction site for an electrode reaction and a diffusion layer for supplying a reactant to the catalyst layer, giving/receiving electrons, and so forth. As the catalyst contained in the catalyst layer, noble metals such as platinum, noble metal alloys, and the like are used in general.
Platinum is widely used as a catalyst for the cathode, since it can attain the highest redox current density among noble metals when forming an electrode and is stable even at a high potential. Alloys of platinum with other metals are widely used as a catalyst for the anode.
However, platinum is expensive and thus is a great obstacle to cost cutting and mass production for wider use of fuel cells. Therefore, catalysts substituting for platinum have been under consideration. For example, Patent Document 1 describes the use of a mixture of a metal complex and a metal oxide as a catalyst for a cathode. It also states that this apparently advances a 4-electron reduction reaction of oxygen. Patent Document 1: Japanese Patent Application Laid-Open No. 2003-151567